The present invention relates generally to underground fluid conduit systems, and more particularly to a novel and improved underground sewer construction and pipe encasement for sewer chimneys, underground pipe lines, and the like.
Great care must be exercised when installing underground sewer systems in wet or unstable soil situations to assure there will be no leakage at the pipe connections after being placed in service. In particular, lateral shifting of the pipe during back filling an excavated site or later, due to flotation or normal settling, an improperly installed system can produce structural cracks or separations in pipe joints. For example, a newly installed heavy concrete sewer chimney may slowly shift under its own weight in loose back fill and eventually separate where it connects to the main sewer line or to upper branch lines. Similarly, installations of long lines of underground pipe, whether or not bedded in crushed stone, are susceptible to cracks and pipe joint separations due to flotation of the pipe or uneven distribution of backfill or later settling of earth around the pipe. Intermittent voids and unsupported spans, so-called "bridging," occur along the pipe and produce shear and bending stresses which can deform bendable pipes out-of-round and produce leaks, especially at bell-and-spigot joints. In consequence, infiltration or exfiltration of unwanted materials occurs and the affected sites must be completely re-excavated, usually at great expense to correct the faults.
Prior art designs of concrete sewer constructions of monolithic poured-in-place castings or of prefabricated sections assembled at the site have been ineffective in minimizing this occurrence. While the latter type is preferred for many installations because the sections are available in various shapes and sizes, and can be assembled in situ to meet specific requirements, the leakage problem is unfortunately exacerbated because the sections may shift among themselves and introduce more separations Furthermore, slight misalignments of sections at each interface forms a potential shelf for debris to build up and clog. A sewer chimney assembled from prefabricated sections, for example, usually includes one or more concrete riser sections of different lengths stacked between a tee-fitting in a lower main line and a lateral service line. To prevent these sections from shifting and separating due to back filling or settling, they are fastened together at the installation site with metal clips, inserts or bolts. This is a costly and labor-intensive procedure, and with time the fasteners may even fail due to metal fatigue, rust and corrosion, or improper installation in general. A practical solution to these problems is needed.